Question

Why do ice cubes always float at the top of a glass of water?

Answer 1

Because water is a very very very very very unusual substance ... It's
the only known substance whose solid form is less dense than its liquid
form near the same temperature.

In other words, water is the only known substance for which a solid lump
of it floats in a liquid glass of it.

If that were not true ... if the behavior of the density of water around its
freezing temperature were the same as the density of all other known
substances ... then life on Earth would be impossible.

Think about that for a while !  Ya gotta admire whoever it was that designed water !

Answer 2

because ice is less dense than liquid water